抄録
The allenic carotenoids, fucoxanthin and peridinin, have been paid much attention as the main light-harvesting pigments in photosynthesis in the sea, because they exhibit high energy transfer efficiencies to Chlorophyll a. This energy transfer efficiency is thought to be related to the intricate structures of these carotenoids. In order to examine their relationship between the unique structures and super ability in carrying out energy transfer, we called to our attention to peridinin, and synthesized peridinin and its derivatives. In addition, ultrafast time-resolved optical absorption and Stark spectroscopic measurements of these peridinin and its modified derivatives have revealed that an intramolecular charge transfer (ICT) state behaves independently from S, state and the S, lifetime of the ICT state converges to a value of 10ア1 ps in methanol for all the peridinin analogues regardless of the extent of a-electron conjugation, and that the allene and C37 carbon skeleton contribute to the generation of a large dipole moment in the excited state of the molecule. A next stage is then to demonstrate the generality of these results by investigating the characteristics of other carotenoids having the ability of high energy transfer efficiency. We focused on fucoxanthin, which possesses the similar ability to peridinin. Fucoxanthin forms the fucoxanthin-chlorophyll a/c-protein (FCP) complex and possesses an allene function and a β,γ-epoxy keto moiety, which is known to be extremely labile to alkali, in a π-electron conjugated system. Only one synthesis of fucoxanthin was reported. The stereochemical control of both the epoxidation and polyene chain formation, however, had not been achieved responsible for this labile moiety. We then started the stereocontrolled synthesis of fucoxanthin, that should be applicable for the syntheses of various kinds of fucoxanthin modified derivatives. Our synthetic strategy had two key points. First, we bisected an allenic segment and a β-ketoepoxy segment to make up the library of each half-segments to synthesize the variously designed fucoxanthin derivatives and coupled them by using the modified-Julia olefination. Second, we intended to realize the satisfactorily stereocontrolled introduction of the cis-epoxide to the C3-a-homoallylalcohol by utilizing the Sharpless epoxidation and then followed by inversion of the resulting unnatural stereochemistry at the C3 hydroxyl group to the corrected one by using the Mitsunobu reaction to control the stereochemistry in the terminal six-membered ring. Herein, we report the stereocontrolled total syntheses of fucoxanthin and polyene chain and allene modified derivatives based on the established method for the natural fucoxanthin synthesis. In addition, ultrafast time-resolved optical absorption measurements were performed on polyene chain modified-fucoxanthin derivatives, which possessed 5 to 8 conjugated olefins. The results showed that the 5, lifetime on fucoxanthin and its derivatives was a similar tendency to that in the case of peridinin. Although the shorter polyene chain derivatives became the longer lifetime in hexane, the S_1 lifetime in methanol converged to a value of 15-22 ps. These results strongly support the unique characteristics of ICT state that we observed in peridinin.
